3GPP Long Term Evolution, LTE, technology is a mobile broadband wireless communication technology in which transmissions from base stations, referred to as eNBs, to mobile stations, referred to as user equipments, UEs, are sent using orthogonal frequency division multiplexing, OFDM. OFDM splits the signal into multiple parallel sub-carriers in frequency. The basic unit of transmission in LTE is a resource block, RB, which in its most common configuration consists of 12 subcarriers and 7 OFDM symbols, which is the same as one slot. A unit of one subcarrier and one OFDM symbol is referred to as a resource element, RE. Thus, an RB consists of 84 REs. An LTE radio subframe is composed of multiple resource blocks in frequency with the number of RBs determining the bandwidth of the system and two slots in time. Furthermore, the two RBs in a subframe that are adjacent in time are denoted as an RB pair. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
The signal transmitted by the eNB in a downlink (the link carrying transmissions from the eNB to the UE) subframe may be transmitted from multiple antennas and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the downlink, a UE relies on reference symbols, RS that are transmitted on the downlink. These reference symbols and their position in the time-frequency grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols.
Messages transmitted over the radio link to users can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each UE within the system. Control messages could include commands to control functions such as the transmitted power from a UE, signaling of RBs within which the data is to be received by the UE or transmitted from the UE and so on.
In LTE Rel-8, the first one to four OFDM symbols, depending on the configuration, in a subframe are reserved to contain such control information. Furthermore, in LTE Rel-11, an enhanced physical downlink control channel was introduced, ePDCCH, in which PRB pairs are reserved to exclusively contain ePDCCH transmissions, although excluding from the PRB pair the one to four first symbols that may contain control information to UEs of releases earlier than Rel-11. FIG. 1 shows a downlink subframe of 10 RB pairs. The subframe is configured with three ePDCCH regions (marked with black) of size 1 PRB pair each. The remaining PRB pairs may be used for PDSCH transmissions.
Hence, the ePDCCH is frequency multiplexed with data messages, i.e. with Packet Data Shared Channel, PDSCH, transmissions contrary to the physical downlink control channel, PDCCH, which is time multiplexed with PDSCH transmissions. Note also that multiplexing of PDSCH and any ePDCCH transmission within a PRB pair is not supported in LTE Rel-11.
Furthermore, two modes of ePDCCH transmission is supported, localized and distributed ePDCCH transmission.
In distributed transmission, an ePDCCH is mapped to resource elements in an EPDCCH set, containing N PRB pairs, where N=2, 4, or 8. In this way, frequency diversity can be achieved for the ePDCCH message. FIG. 2 shows a downlink subframe with 4 parts belonging to an ePDCCH. The parts are mapped to multiple of the enhanced control regions known as PRB pairs, to achieve distributed transmission and frequency diversity.
In localized transmission, an ePDCCH is mapped to one PRB pair only, if the space allows, which is always possible for aggregation level one and two and for normal subframes and normal Cyclic Prefix, CP, length also for aggregation level four. In case the aggregation level of the ePDCCH is too large, a second PRB pair is used as well, and so on, using more PRB pairs, until all enhanced Control Channel Elements, eCCE, belonging to the EPDCCH has been mapped. FIG. 3 shows a downlink subframe where the 4 eCCEs belonging to an ePDCCH is mapped to one of the enhanced control regions, to achieve localized transmission.
To facilitate the mapping of eCCEs to physical resources each PRB pair is divided into 16 enhanced resource element groups and each eCCE is split into L=4 or L=8 enhanced Resource Element Groups, eREGs, for normal and extended cyclic prefix, respectively. An ePDCCH is consequently mapped to a multiple of four or eight eREGs depending on the aggregation level.
These eREGs belonging to an ePDCCH resides in either a single PRB pair, as is possible for localized transmission, or a multiple of PRB pairs, as is possible for distributed transmission. The division of a PRB pair into eREGs is illustrated in FIG. 4, which shows a PRB pair of normal cyclic prefix configuration in a normal subframe. Each tile is a resource element where the number corresponds to the eREG it is grouped within. The marked REs with 0, corresponds to the REs belonging to the same eREG indexed with 0.
Furthermore, how L=4 or L=8 eREGs respectively are grouped into the eCCEs is described in [3GPP TS 36.213].
The ePDCCH resources may be UE specifically configured in terms of ePDCCH sets. An ePDCCH set is a collection of N PRB pairs containing 16N/L eCCE with the possible values of N=2, 4, 8. A UE can be configured with K=1 or K=2 sets simultaneously and where the value N can be different for each of the K sets. Each set may also be configured to be of either localized or distributed type. For example, a UE may be configured with K=2 and N1=4 and N2=8 and where the first set is used for localized transmission and the second for distributed transmission. The total number of blind decodes, 32 in the case uplink multiple-input multiple-output, MIMO, is not configured, is split between the K sets. How this split is done is described in 3GPP [TS 36.213]. Hence, a UE will monitor Bi ePDCCH candidates in ePDCCH set i.
Each ePDCCH consists of AL eCCEs where AL is the aggregation level of the message. Each eCCE in turn consists of L eREG where L=4 or L=8. An eREG is a group of RE which is defined in 3GPP specification TS 36.211. In each PRB pair there is 16 eREG. When ePDCCH collides in mapping with own cell Cell-specific Reference Signal, CRS, or own cell legacy control region, these signals have priority and ePDCCH is mapped around these occupied REs and code chain rate matching is applied. This means that the effective number of available RE per eREG is usually less than the 9 RE but there is no interference from the own cell CRS or own legacy control region signals since the ePDCCH is mapped around these signals.
The cell-specific reference signal, also known as the common reference signal, is broadcasted periodically by LTE systems to provide a UE the ability to measure the channel used for certain downlink transmissions. The CRS is, for example, used to demodulate the Physical Broadcast Channel, PBCH, but also to demodulate the PDSCH for, for example, transmission modes 1-4, which are the transmission modes that are primarily used for communication to any LTE Rel-8 and Rel-9 UE. For these transmission modes, the CRS is also utilized for the purpose of channel state information, CSI, measurements, which are reported to the network for improved link adaptation and MIMO downlink processing. Another application of CRS is for mobility measurements.
Between cells, the CRS may be shifted in frequency domain. This is often used in real-life deployments including conventional homogenous deployments with macro nodes.
The different antenna ports of the CRS are mapped to different sets of resource elements in the grid. Moreover, for all resource elements assigned to a CRS port, the corresponding resource elements may be muted, zero-power, on all other antenna ports. The overhead of the CRS thus increases with increasing number of transmitter antenna ports, 8, 16, and 24 resource elements per PRB pair, for 1, 2 and 4 antennas, i.e. CRS antenna ports, respectively.
The same enhanced control region, see for example FIG. 3, can be used in different transmission points within a cell or belong to different cells that are not highly interfering with each other.
To reduce interference between different transmission points, various interference coordination techniques may be used, such as enhanced Inter-cell interference coordination, eICIC, or Coordinated Multi Point, CoMP, operation introduced in LTE Rel-11.
A heterogeneous network comprises a number of low-power network nodes and a number of high-power network nodes, which coverage areas may overlap each other partially and/or totally. A low-power network node is a node providing coverage to a small area, such as a pico node, e.g. a pico eNB. A high-power network node is a node providing coverage to an area larger than the small area, such as a macro node, e.g. a macro eNB. To increase the UE pick-up area of a low-power node (i.e., the area in which a UE would connect a pico node rather than a high power macro node), cell range expansion, CRE, is a powerful tool where a UE is prevented to make a handover to the high-power node unless the received power from the high-power node exceeds the received power of the low-power node by a configured CRE margin. This effectively increases the “coverage area” of the low-power node. However, for UEs in the so-called cell-range expansion area, i.e., the area where UEs connect to the low-power node, but signals from the high-power node are received with a stronger power than signals from the low-power node, it is advantageous that the high-power node minimizes the interfering signals in the subframes where the network communicates with these UEs.
However, not all interference from the high-power node can be muted in a subframe, such as the transmission of the CRS. In particular, for cell-range expansion UEs to be able to accurately estimate a propagation channel based on the CRS transmitted by the low-power node, it is advantageous that the CRS of the macro node does not collide with the CRS of the low-power node. This can be ensured by configuring different CRS shift in frequency of the high-power node and the low-power node.
Today, mapping of ePDCCH is performed such that the ePDCCH is mapped around other signals, e.g. CRS or CSI-RS, of the same cell as in which the ePDCCH is distributed, i.e. serving cell. In other words, the resource elements, REs, used by the ePDCCH are not coinciding with the REs used by the other signals of the same cell. Thereby, there is no collision of the ePDCCH with the CRSs of the same, serving, cell. The UE is implicitly informed on which REs the other signals are situated. As an example, CRS positions are given by the Cell-ID and CSI-RS is given by UE specific signaling using the RRC protocol. However, it has been discovered that there are use cases where other mappings may be needed, where REs different than those occupied by the CRS and CSI-RS transmitted by the serving cell need to be mapped around. For example, in heterogeneous networks using CRE, a UE may be situated in the CRE area and connected to a low-power node, and experience high interference from a signal of a high-power network node. In that case, the signal of the high-power node may need to be avoided in the ePDCCH mapping in the serving cell of the low-power node, but if the UE is situated closer to the low-power node, the signal from the low-power node is the strongest and needs instead to be avoided.